CYTOSOLIC PROTEOLYSIS OF TAU BY CATHEPSIN-D IN HIPPOCAMPUS FOLLOWING SUPPRESSION OF CATHEPSIN-B AND CATHEPSIN-L

Incubation of cultured hippocampal slices with an inhibitor [N-CBZ-L-p henylalanyl-L-alanine-diazomethyl ketone (ZPAD)] of cathepsins B and L resulted in the degradation of high molecular weight isoforms of tau protein and the production of a 29-kDa tau fragment (tau(29)). A tau a ntibody that is sensitive to the phosphorylated state of its epitopes did not recognize tau proteins or the tau(29) fragment in slices that had been treated with a protein phosphatase inhibitor. This strongly s uggests that the tau fragment was located in an extralysosomal compart ment accessible to kinases and phosphatases. tau(29) exhibited a signi ficant capacity for binding to microtubules and thus has the potential for interfering with normal tau-tubulin interactions. Three lines of evidence indicated that ZPAD-induced tau proteolysis was mediated by c athepsin D: (a) slices treated with the inhibitor had markedly elevate d levels of cathepsin D in both lysosomal and extralysosomal compartme nts; (b) co-incubation of cathepsin D and tau at neutral pH resulted i n a loss of intact tau proteins and production of a 29-kDa fragment; a nd (c) the lysosomotropic drug chloroquine blocked ZPAD-induced increa ses in mature cathepsin D, and this was accompanied by a suppression o f ZPAD-induced tau proteolysis. Changes in lysosomal hydrolases and cy toskeletal perturbations occur during brain aging. The present results suggest that the enzymatic and structural effects are related and, mo re specifically, are linked by alterations in the concentration and lo calization of cathepsin D. The tau fragments with microtubule binding capacity generated by cathepsin D could also be a source for the small polypeptides found in association with age-related pathological featu res.